It is well known that smoking articles, particularly conventional filtered or unfiltered cigarettes, provide an increasing per puff yield of particulate matter ("puff-to-puff yield") as the cigarette is smoked. In recent years, high efficiency filters and air dilution have been used to provide a lower total yield of particulate matter in the so-called "low tar" and "ultra low tar" cigarettes now available in the marketplace. Such high efficiency filters significantly increase the pressure drop of the cigarette and decrease yield, especially in the first few puffs of the cigarette. Air dilution helps to reduce the pressure drop somewhat, but also further reduces the per puff yield in the first few puffs. In combination, high filtration efficiency and air dilution configured in the conventional way in a cigarette not only produces the desired lower total yield, but also produces an undesirable per puff yield or puff profile of little yield in the first few puffs and a high yield in the final few puffs. Such a puff profile results in a perception by the smoker of a cigarette with inconsistent taste characteristics, i.e., little or no taste in the first few puffs and a harsh and overbearing taste in the final few puffs.
The prior art has attempted to address this problem in several ways. In one known construction of a cigarette smoking article, a compound filter is made up of two filter segments, a rod end segment and a mouth end segment. The rod end segment is made of a cellulose acetate fiber tow with a relatively high pressure drop thereacross (high efficiency) and which has been provided with one or more hollow cellulose acetate fibers or capillary tubes extending therethrough. The mouth end filter segment abuts the rod end filter segment and is made of a cellulose acetate fiber tow with a relatively low pressure drop thereacross (low efficiency). Air ventilation holes are provided in the rod end filter segment.
In a cigarette of the foregoing construction, during the first few puffs, a major portion of the smoke travels from the lighted end of the cigarette through the tobacco rod, through the capillary tube(s) in the rod end filter segment and thence through the low efficiency mouth end filter segment. This results in relatively unfiltered smoke, i.e., smoke with greater particulate matter and a better taste, reaching the smoker. A relatively small volume of smoke and air travels through the high efficiency filter and air ventilation holes of the rod end segment during the first few puffs because the pressure drop across the two filter elements is higher than the pressure drop through the capillary tube(s). As the smoker continues to smoke the cigarette, particulate matter or "tar" begins to accumulate at the mouth end of the capillary tube(s) where that end abuts the mouth end filter segment. Such accumulation is supposed to block the capillary tube(s) thereby increasing the pressure drop through the tube(s) and causing more of the higher yield smoke to flow through the high efficiency, air diluted filter surrounding the tubes. During the final few puffs when the per puff yield is highest, the capillary tubes are supposed to be substantially blocked so that most of the high yield smoke from the tobacco rod is more efficiently filtered and air diluted to maintain the per puff yield at a level substantially the same as the per puff yield of the first few puffs. The result is a cigarette with a substantially constant per puff yield.
It has been noted, for example, in European Patent Publication No. 0 481 596 that cigarettes having the above-described filter construction may exhibit significant variation in smoking characteristics which may render the cigarette commercially unacceptable. That publication suggests a modification of the known constructions by the addition of a third filter segment between the tobacco rod and the filter segment which carries a perforated capillary tube to achieve more consistent smoking characteristics.
U.S. Pat. Nos. 4,460,001 and 4,469,112 disclose a compound cigarette filter construction which is said to provide a substantially constant nicotine and tar delivery. A barrier positioned upstream of a filter segment is provided with one or more passageways through which the smoke passes. Buildup of tar in the filter segment downstream of the barrier results in an increasing blockage of flow which results in increased pressure drop and filtration efficiency and thereby yield a substantially constant nicotine and tar delivery.
U.S. Pat. Nos. 4,393,885 and 4,585,015 disclose a cigarette filter formed of an air or air ventilated pervious filter rod of cellulose acetate tow or the like with a central channel and a gas pervious partition disposed between the filter segment and the tobacco rod. An orifice in the partition communicates with the central channel and is gradually blocked by particulate matter during smoking to thereby increase the pressure drop and air ventilation resulting in a more constant per puff delivery of particulate matter.
Similar filter constructions for achieving a more constant per puff yield of particulate matter are disclosed in U.S. Pat. Nos. 4,291,712 and 4,942,887 and British Patent No. 1,428,018.
One of the drawbacks of the prior art constructions is the inconsistency of results which makes it virtually impossible to design and manufacture a commercially acceptable cigarette product based on the specifications and data provided in the prior art disclosures. An important reason for the inconsistency of results in the prior art constructions is believed to reside in the fact that compound filter constructions are subject to a large number of variables, such as the pressure drop across the individual filter segments, the pressure drop across the abutment or interface between filter segments, the pressure drop of capillary tubes used in the filter segments, the total pressure drop of the combination of filter elements, the amount of air ventilation, the location of the air ventilation holes, etc. One of the variables that is more difficult to control and is believed to account for much of the inconsistency of results of the above-described compound filter is the abutment resistance or pressure drop between the rod end and mouth end segments. This pressure drop has not been adequately accounted for in the prior art.
In addition, the development of a cigarette filter with a predetermined puff profile is usually based on a trial-and-error experimental approach. Even if a reasonably constant per puff yield can be achieved with a particular compound filter construction designed according to such an approach, if it is desired to change the total yield of particulate matter to a higher or lower level, the further changes that are necessitated in the filter design parameters to achieve that level are likely to also alter the puff profile of the cigarette requiring further trial-and-error experimentation.
It would be desirable therefore to develop models of a compound filter construction that would provide the key specifications for a filter construction based on a desired total yield of particulate matter, e.g., WTPM, within a range of WTPM and a desired puff profile from a constant or level per puff yield to a substantially decreasing per puff yield.